The present invention relates to a device for controlling the rotation speed of a motor of the kind requiring variable rotation speed control over a relatively wide range, e.g. a DC motor adapted to drive a scanner of a copier on which an optical arrangement is mounted.
Today, various kinds of motors are used to drive various kinds of objects. For example, a DC motor is installed in a copier to drive a scanner which carries therewith optics for imaging at a predetermined speed in a predetermined direction. Many of modern copiers are furnished with a capability for enlarging or reducing the size of images to be copied. In a copier, the prerequisite for the variable magnification capability is that the moving speed of the scanner and, therefore, the rotation speed of the motor for driving the scanner be variable. Usually, the variable range of the motor speed necessary for variable magnification is expressed as: EQU V.ltoreq.v.ltoreq.4V Eq. (1)
where v is a motor speed and V the lowest motor speed.
It will be seen from the Eq. (1) that the motor speed varies over a substantial range and, therefore, it needs to be set up in such a manner as to cover such a wide range.
The rotation of the motor is transmitted to the scanner and transformed into a linear motion of the latter by a mechanism which usually is made up of a gear, a pulley, a wire and others. The problem with those structural elements is that scattering is unavoidably introduced in the production stage or the assembly stage and due to wear which is atrributable to aging. Such scattering causes one scanner to move at a different speed from another even if the motor rotation speed is the same. To compensate for the scattering, motor rotation has to be finely adjusted.
It has been customary to implement the fine motor speed adjustment by varying the set speed of the motor. Where the magnification of images is changed on a 1% bases, for example, the set speed of the motor may also be varied on a 1% basis within the variable range as defined by the Eq. (1). However, where it is desired to change the magnification on a smaller order such as 0.1%, it is necessary for the set speed of the motor to be finely controlled on a 0.1% order within the defined variable range. Hence, the computation of a motor speed, proportional integration (PI) and others for the motor speed control have to be performed with accuracy which is great enough to follow such fine adjustment. This in turn requires intricate circuits, renders the control unstable, and needs readjustment to compensate for aging. Further, the motor speed has to be compensated every time it is accidentaly changed.